112 THE VOYAGE OF H.M.S. CHALLENGER. 



This upper extremity and the base are slightly enlarged, otherwise the peduncle is of 

 the same thickness throughout. 



The branchial aperture is situated on the dorsal edge near the anterior end, and about 

 one-fourth of the distance from the point of attachment of the peduncle to the posterior 

 end. It is very conspicuous, and projects slightly from the surface of the test. It has 

 the form of a transverse slit, large and open, and having its posterior edge arched so as 

 to form an approach to the triangular outline seen in the preceding species. The lips are 

 large and prominent, especially the posterior one, but smooth ; no papillary fringe is present. 

 The branchial aperture is directed anteriorly and slightly dorsally (PI. XL fig. 10). 



The atrial aperture is not very distant from the branchial, being situated at the 

 junction of the dorsal edge with the rounded posterior end, and is directed dorsally and 

 posteriorly. It is conspicuous, but not so prominent as the branchial aperture, is of 

 moderate size, and gapes slightly. It has the form of a transverse slit bounded by raised 

 anterior and posterior lips which are both smooth. 



The surface is regular, but finely granular from the presence of minute processes all 

 over, except on the lips of the apertures which are perfectly smooth. The surface of the 

 peduncle and its continuation along the dorsal edge are also smooth. 



The colour is a light grey, with a translucent appearance. The peduncle is hyaline 

 and transparent. 



The dimensions are as follows : — 



TJie Test, like that of Culeolus recumbens, is thin and flexible, but tough. Unlike it, 

 however, it is transparent. It is very compact, and in minute structure resembles that 

 of the last species. The matrix is very close and homogeneous, and contains only rounded 

 and elongated protoplasts, generally arranged parallel to the surface. In the sections 

 examined no vessels were present, but every here and there along the inner surface small 

 pouches or excavations in the matrix were discovered containing blood-corpuscles (PI. 

 XII. fig. 8, v.). These are doubtless diverticula from the sinuses of the mantle. 



The outer surface is rough from the presence of minute processes rising occasionally 

 into small papillas which may equal the thickness of the test in height (PL XII. fig. 

 8, t.p). These have generally sand-grains and minute particles of mud adhering to 

 them, but the general surface is not coated as in the preceding species. 



The peduncle is hollow. It is a thin-walled tube, the thickness of the wall being 

 only about one-sixth of the total diameter. The interior of the tube is lined by a 

 membrane formed of tesselated epithelial cells, diamond-shaped or short fusiform, and 



